1. Field of the Invention
The present invention generally relates to micromachined sensors. More particularly, this invention relates to a process for forming a monolithically-integrated sensor comprising a micromachined transducer and sensing circuitry combined on a single silicon substrate.
2. Description of the Related Art
Integrated micromachined sensors are generally fabricated using a post-processing approach, in which the micromachined features are formed by etching after the processing circuitry is fabricated. Wet anisotropic etch techniques have typically been used to define recesses and release membranes of micromachined features. However, wet anisotropic etching requires significant horizontal margins because etching occurs along the planes of the silicon wafer at a 54.7 degree angle. As a result, die size must be increased to allow for sufficient device tolerances, with the disadvantage that integrated micromachined sensors are not as compact as might be desired.
Another limitation associated with existing micromachined sensors integrated with CMOS (complementary metal oxide semiconductor) and BiCMOS (bipolar and complementary metal oxide semiconductor) processes is that dielectric layers utilized in such processes are in compression due to adhesion requirements on metal layers and long-term reliability. However, there exists the potential for significant yield loss in dielectric isolated structures, such as micromachined diaphragms, due to wrinkling caused by the compressive stresses within such dielectric layers.
The present invention is a process using integrated sensor technology in which a micromachined sensing element and CMOS or BiCMOS signal processing circuits are combined on a single semiconductor substrate, in which the process steps provide a more compact sensor and improved yields as compared to previous integrated micromachined sensors. The process is based on modifying a BiCMOS or CMOS process to produce an improved layered micromachined member, such as a sensor diaphragm, after the circuit fabrication process is completed. Compressive stresses within the composite layer of the micromachined member are significantly reduced or eliminated to improve yields. The process is well suited for the fabrication of micromachined thermopile transducers for use as infrared sensors, though other types of micromachined sensors are foreseeable and within the scope of this invention.
Generally, the process of this invention entails forming a circuit device on a substrate by processing steps that include forming multiple dielectric layers and at least one conductive layer on the substrate. The multiple dielectric layers comprise an oxide layer on a surface of the substrate and at least two other dielectric layers that are in tension, with the conductive layer being located between the two dielectric layers. The surface of the substrate is then dry etched to form a cavity therein and thereby delineate a micromachined member and a frame surrounding the micromachined member. The dry etching step terminates at the oxide layer, such that the micromachined member comprises the multiple dielectric layers and the conductive layer.
As described above, the process of this invention is able to produce a sensor characterized by reduced signal noise as a result of the sensing (micromachined) member being fabricated on the same chip as its signal processing circuitry, thereby minimizing the distance that the transducer signal must be transmitted. Fabrication of the sensor structure does not require high dopant concentrations, thermal treatments or other processing steps that would be incompatible with standard BiCMOS and CMOS devices, such that the signal processing circuitry can make use of CMOS and BiCMOS technology. The sensor also does not require the use of materials and process steps that are not conducive to mass production processes made possible with CMOS technology.
In addition to the above, the process of this invention results in stresses within the deposited layers being effectively tensile after the completion of the IC fabrication process. More particularly, the process of this invention forms tensile films both above and below the conductive layer to provide good adhesion while converting to tensile the net stress in the composite dielectric stack, such that the potential is reduced for yield losses attributable to compressive stresses within the dielectric stack. According to another aspect of the invention, the dry etch provides various advantages, including producing walls normal to the etched surface so as to reduce the size of the die required to accommodate the integrated micromachine.
Other objects and advantages of this invention will be better appreciated from the following detailed description.